1. Field of the Invention
The present invention relates to light emitting devices and more particularly to light emitting devices having luminescent layers comprising piezoelectric material.
2. Description of the Related Art
Piezoelectric thin-films have been widely used in vibrators, such as piezoelectric resonators and piezoelectric actuators, and driving devices. In recent years, piezoelectric thin-films also have attracted attention as optical devices. For example, Japanese Unexamined Patent Application Publication No. 7-262801 discloses a light emitting device which has a ZnO film formed on a sapphire substrate and emits ultraviolet light by the effects of excitons. Moreover, Japanese Unexamined Patent Application Publication No. 10-256673 discloses a light emitting device which emits ultraviolet light by laser oscillation.
Properties of piezoelectric films, however, have not been sufficiently known. In particular, properties of piezoelectric films suitable for light emitting devices and methods for making the piezoelectric films have not yet been sufficiently studied. Thus, piezoelectric films used in conventional light emitting devices do not have high luminous efficiency.
Accordingly, it is an object of the present invention to provide a light emitting device having high luminous efficiency and high luminous intensity. The light emitting device comprises: a substrate; and a first piezoelectric film held on the substrate and having one of a positive plane and a negative plane, the first piezoelectric film functioning as a light emitting layer.
According to another aspect of the present invention, a method for making a light emitting device comprises the steps of: preparing a substrate; and forming a first piezoelectric film having one of a positive (+) upper plane and negative (xe2x88x92) upper plane on the substrate, according to properties of the substrate.
When the first piezoelectric film has the positive plane, it is preferable that the substrate is selected from the group consisting of a c-plane sapphire substrate, an R-plane sapphire substrate, an m-plane sapphire substrate, an X-plane sapphire substrate, an a-plane sapphire substrate, a rotated Y-cut plate sapphire substrate, a double rotated sapphire substrate, a rotated Y-cut plate quartz substrate, a Z-plane quartz substrate, a LiTaO3 substrate having a negative Z plane of a rotated Y-cut plate, and a LiNbO3 substrate having a negative Z plane of a rotated Y-cut plate.
When the first piezoelectric film has the negative plane, it is preferable that the substrate is selected from the group consisting of a LiNbO3 substrate having a positive Z plane of a rotated Y-cut plate LiNbO3 substrate, a LiTaO3 substrate having a positive Z plane of a rotated Y-cut plate LiTaO3 substrate, a LiTaO3 substrate having a positive Z plane, a LiNbO3 substrate having a positive Z plane, a glass substrate, A Si substrate, a metal substrate, and a substrate having a metal film thereon.
The first piezoelectric film preferably comprises a material selected from ZnO, AlN, and CdS.
The light emitting device may further comprises a second piezoelectric film, the first piezoelectric film and the second piezoelectric film having different conductivity types.
In the light emitting device, the substrate may have a plurality of metal film stripes thereon, the first piezoelectric film covers the metal film stripes, and the first piezoelectric film has the negative plane.
According to the present invention, a piezoelectric film having a positive plane or a negative plane is selectively provided depending on the type of the substrate, and the piezoelectric film has high crystallinity. Thus, the resulting light emitting device has high brightness and high luminescent efficiency.